This invention relates to the field of engraving machines.
Engraving with a computer numerically controlled (cnc) type milling machine is typically performed by the following method. A pointed engraving tool is held in a rigid collet or endmill holder and rotated in the spindle of a cnc milling machine. The cnc milling machine controls the depth of engraving by plunging the pointed engraving tool down into the workpiece by a fixed amount. The cnc milling machine then moves the engraving tool across the surface of the part to be engraved at that specific depth. If the material to be engraved is not perfectly flat on the cnc machine table, the engraving will not look even. Some areas of the engraving will be deeper than others and some areas may not be engraved at all.
To overcome this problem, a floating spring loaded engraving tool that is held in a collet and rotated in the spindle of a cnc milling type machine may be used. Such a tool is disclosed and claimed in Nelson U.S. Pat. No. 6,834,434 issued Dec. 28, 2004 and is incorporated by reference herein. A spring provides the pressure to push the engraving tool against the workpiece being engraved. The depth of engraving is controlled by both the strength of the spring and the feedrate of the tool across the surface of the workpiece. If the rate of travel of the engraving tool across the surface being engraved is slow, the engraving tool has more time to press against the workpiece and therefore plunges deeper into the surface. Speeding up the feedrate of the engraving tool over the surface of the workpiece allows less time for the tool to plunge into the surface so shallower engraving is produced. Using this method of adjusting the feedrate to control the depth of engraving is effective; however a technique to provide absolute depth control is often desired for the following reasons.
When the spring loaded engraving tool is used to engrave very soft materials such as plastic, there is a possibility that the engraving tool will penetrate too deeply when the tool begins its first cut. The cnc machine moves the tool to the beginning of the line to be engraved and plunges the tool to the specified depth. The tool is then moved across the surface of the workpiece to be engraved at a specific feed rate. When the tool is moved across the surface of the work piece at a relatively fast feed rate, the depth of the engraving will not be the full depth that the cnc machine has plunged the tool. The engraving tool will not have sufficient time to plunge all the way down to the specified depth. The spring in the Spring Loaded Engraving Tool will compress and the engraving tool bit will skim across the surface at a depth less than the full depth specified by the cnc machine. If the plunging feed rate at the start of each line is not fast enough and the material is very soft, the engraving tool bit will have sufficient time to plunge deeper into the surface of the work piece than the amount that the tool plunges into the surface when it is traveling over the surface of the work piece. The engraving tool bit will essentially be drilling down into the work piece at the start of each line that is engraved. This will cause the appearance of each engraved line to have a circle or small hole at the starting point. It is desirable to prevent the engraving tool bit from being able to plunge too deeply into the workpiece at the start of each engraved line to ensure that all the lines will be consistent.
Manufacturers such as Gravograph (Duluth, Ga.) and Vision Computerized Engraving Systems Phoenix, Ariz.) have developed engraving spindles that incorporate a device for controlling the depth of engraving and use a spring for pressing the engraving tool against the workpiece to be engraved. These engraving spindles are integrated components of the cnc engraving machines and are not designed to be held in a collet or endmill type holder and placed into a rotating spindle of a cnc machine tool such as a milling type machine. These engraving machines have their own motor therein for rotating the spindles carrying the rotating engraving tool bits. An integral nosepiece can be placed onto the bottom of the engraving spindle to position a given length of the engraving tool that protrudes from the end of the engraving spindle, but the nosepiece must be attached to a non-rotating portion of the spindle coupled to the cnc machine. Furthermore, the nosepiece does not function as a means for holding a collet for gripping the engraving tool and does not rotate, and also the mechanism that allows the engraving tool to float is unable to rotate and must be held stationary with respect to the cnc machine.
These engraving spindles have a very long engraving tool inserted into them from the top side of the spindle opposite the bottom side adjacent the tool point, i.e. the engraving tool is inserted into the top of the spindle instead of the bottom. Thus the engraving tool must be longer than the length of the entire spindle so it can protrude out of the bottom of the spindle. A knob with a set screw on the top of the rotating inner spindle is the means for holding and vertically positioning the engraving tool over the workpiece. The long engraving tool must be supported along its length to prevent wobbling or whipping of the tool. This is accomplished by passing the engraving tool through a long precision diameter hole in the inner spindle that supports the engraving tool at both ends. The nosepiece does not provide any engraving tool holding mechanism.
A significant problem with this type of top loading engraving spindle is that when the engraving tool dulls and needs to be replaced, the entire process for setting the length of the tip protruding from the end must be performed again. There is no repeatable means for easily setting the tool length. Additionally, the long engraving tools are more costly to produce.
Kavo (Leutkirch, Germany) developed an engraving spindle that incorporates an integral motor with a mechanism that allows floating of the engraving tool. They incorporated a collet and collet nut that are attached directly to the rotating motor shaft. The housing that incorporates the floating mechanism is clamped to the cnc machine and the entire motor, motor shaft, collet, and engraving tool are able to float. The nosepiece is attached to the non rotating floating mechanism. This design could allow repeatable locating of the engraving tool when a dull engraving tool is replaced by a sharp one, but does not allow the entire floating system to be held in a rotating collet or endmill holder of a cnc milling type machine. Therefore the floating tool may not easily be removed from the cnc machine. This is a serious detriment to the manufacturing process. If multiple type tools are needed to produce a part, for example, if a part needs to be drilled with two drills with different diameters and then engraved, it would not be possible with either of the above designs.
The spring loaded engraving tool of the aforesaid Nelson U.S. Pat. No. 6,834,434 overcomes this major problem by being able to be held by a collet or endmill holder and placed into the rotating spindle of a standard milling type machine. It has a collet holder with a short hole on the end for receiving a collet and an engraving tool. The collet and engraving tool are inserted into the hole until they reach the bottom. A collet nut with an integral nosepiece is threaded onto the collet holder of the spring loaded engraving tool to compress the collet and firmly hold the engraving tool. Different diameter collets and engraving tools can be inserted into the collet holder. Since the engraving tools are all made to the same length, it is not necessary to reset the nosepiece to adjust the length of the engraving tool tip protruding from the end as discussed hereinabove.